BIL having an affair

My sister has never been happy in her marriage, but stuck with it due to having a small child. Finally at the start of the year they decided to get divorced and had agreed to stay under the same roof until they sold their house. There has been evidence of an affair for a while but DS has chosen, for her own reasons to ignore it. Finally today she has proof he is having an affair, which he says has only been going on for two weeks ( which is certainly not true ), is there anyway she can get him out of the house, as obviously the situation is unbearble for her. My BIL does next to nothing for his dc, leaving my DS to do everything, he is a mental bully, brought about by serious self esteem issues. Their divorce is underway, but could take 6 months and my sister really cannot tolerate living under the same roof as him, now she knows he has been unfaithful. He has family close by, whilst my sister does not, so could move it with them, particularly a brother, who lives alone....Any advice appreciated. Thanks